As a substitute for transparent flat glass, transparent materials having non-shattering nature or greater shatter resistance than glass have been widely used in these years. For example, plastic substrates, especially polycarbonate resins, due to their excellent properties including transparency, impact resistance, and heat resistance, are currently used as structural members in place of glass, in various applications including windows in buildings and vehicles, meter covers and the like.
However, it is strongly desired to improve the surface properties of molded polycarbonate resins because their surface properties like mar resistance and weatherability are poor as compared with glass. Nowadays, polycarbonate resins intended for use as vehicle windows, road noise barriers or the like are desired to withstand weathering over ten years.
Known means for improving the weatherability of molded polycarbonate resins include lamination of an acrylic resin film having good weatherability to the surface of polycarbonate resin substrates, and provision of a UV absorber-containing resin layer on the polycarbonate resin surface by coextrusion or the like.
Also, known means for improving the mar resistance of molded polycarbonate resins include coating of thermosetting resins such as polyorganosiloxane and melamine resins and coating of polyfunctional acrylic photo-curable resins.
Further, a method of producing a transparent part having both weatherability and mar resistance is disclosed in JP-A S56-92059 and JP-A H01-149878. UV-absorbing transparent articles are known comprising a resin substrate, an undercoat layer having a large loading of UV absorber added, and a protective coating of a colloidal silica-laden polysiloxane coating composition formed on the undercoat layer.
However, the heavy loading of UV absorber into the undercoat layer gives rise to several problems. The heavy loading can adversely affect the adhesion of the undercoat layer to the underlying substrate or the overlying protective coating of a colloidal silica-laden polysiloxane coating composition. The UV absorber will escape from the undercoat composition via volatilization during heat curing step. On outdoor use over a long period of time, the UV absorber will gradually bleed out, exerting detrimental effects like cracking and whitening or yellowing. It is unacceptable from the standpoint of mar resistance to add a large amount of UV absorber to the colloidal silica-laden polysiloxane coating composition of which the overlying protective coating is made.
It is known from JP-A H08-151415 to form a protective coating on the surface of a synthetic resin or the like using a mixture of a benzotriazole or benzophenone-derived UV-absorbing vinyl monomer and a vinyl monomer copolymerizable therewith as a coating component. This protective coating, however, has only limited mar resistance since it is made of a vinyl polymer.
It is also known to form a coating on a resin substrate to produce a multilayer resin article using a copolymer of a benzotriazole or benzophenone-derived UV-absorbing vinyl monomer, an alkoxysilyl-containing vinyl monomer, and a vinyl monomer copolymerizable therewith as a coating component, the coating having adhesion to the resin substrate and imparting weatherability to the article. See JP-A 2001-114841, JP 3102696, JP-A 2001-214122, JP-A 2001-047574, JP-A 2007-231060, and JP-A 2008-120986.
In these patent documents, coated articles having mar resistance and weatherability are manufactured by using a copolymer-containing composition to form an undercoat, and forming a colloidal silica-laden polysiloxane resin coating on the undercoat. These articles are noticeably improved in the adhesion of the polysiloxane resin coating and weatherability. Since the crosslinking network formation of alkoxysilyl groups in the undercoat does not proceed to a full extent, post-crosslinking of residual (or uncured) alkoxysilyl or hydroxysilyl groups can occur with the passage of time, inviting a likelihood of introducing strain to the coating and thus causing defects like cracks and delamination. That is, the coated articles are still insufficient in long-term weathering. In addition, when the coating is exposed to rapid changes of the ambient temperature, especially changes at relatively high temperature, the likelihood of crack occurrence by post-crosslinking is increased.
It is also known to use zinc or titanium oxide nanoparticles as the metal oxide nanoparticles having UV shielding property (see JP 3319326, JP 3846545, JP-A H11-209695, JP 3347097, and JP-A 2002-60687). However, on account of residual photocatalytic activity, a coating loaded with such metal oxide nanoparticles can not avoid a phenomenon that the coating develops cracks or peels in a weathering test.
JP 3509749 and JP-A 2002-87817 disclose an attempt to suppress photocatalytic activity by coating surfaces of zinc or titanium oxide nanoparticles with another oxide. A coating loaded with surface-coated zinc oxide nanoparticles has a longer lifetime in a weathering test than bare zinc oxide nanoparticles. However, the coating is still insufficient as outdoor UV shielding members partly because cracks develop in a long-term weathering test.
In general, visible light transparency is one of important properties of coating compositions for forming weather resistant surface protective coatings. If metal oxide nanoparticles are used as the UV shielding agent, visible light transparency is substantially impaired depending on an average particle size and a tendency to agglomerate. JP-A H11-278838 discloses that when zinc oxide nanoparticles are prepared by a specific method, a dispersion thereof has a smaller particle size and is unsusceptible to agglomeration. A coating composition having this zinc oxide nanoparticle dispersion compounded therein would be highly transparent to visible light although this is not described in Examples.
As discussed above, a number of attempts have been made to improve the weather resistance, mar resistance and other properties of coating compositions. However, there is not available a coating composition whose cured film exhibits mar resistance, UV shielding property, and sufficient weather resistance and durability to withstand prolonged outdoor exposure while maintaining visible light transparency.